Global variable support for retry clients. Normally it is not necessary for
clients to be aware of the surrounding environment because a
RetryCallback can always use the context it is passed by the
enclosing RetryOperations. But occasionally it might be helpful to
have lower level access to the ongoing RetryContext so we provide a
global accessor here. The mutator methods (clear() and
register(RetryContext) should not be used except internally by
RetryOperations implementations.